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2012 C not starting right away

Discussion in 'Prius c Care, Maintenance and Troubleshooting' started by brianhjazz, Jul 30, 2023.

  1. brianhjazz

    brianhjazz New Member

    Joined:
    Jul 30, 2023
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    Location:
    MN, USA
    Vehicle:
    2012 Prius c
    Model:
    Two
    Hey all, recent first time hybrid owner here - bought a used 2012 C Two a few months ago and have loved it so far! Honestly drives quite smooth for a tiny car, I can fit a ton of stuff in the back with just half of the rear seat folded down, and of course the mileage is awesome!

    This past Thursday, middle of the afternoon (95+ degrees with a heat index of 105), I went out to run a few errands and found what I assumed to be a very dead 12v battery in my C - the key fob wouldn't even unlock the doors, and none of the lights or the display came on once I manually opened it. After a few moments of "oh $***", I realized the dome lights and the display were starting to come on, so I opened everything to let the interior cool off hoping the 12v was maybe just overheated, and lo and behold after about 5 minutes the car started right up! Drove around town both Thurs and yesterday (multiple trips both days) and had no issues at all. Fast forward to today (Sat), mid-morning (about 70 degrees out and parked in the shade), my car did the exact same thing. Not even enough power to work the power door locks, and after about 5 minutes the car started right up and I had 0 issues throughout the rest of the day again.

    I'm definitely not a car guru, but I do know a few things having owned and been around my share of clunkers throughout my years - I plan to borrow my friend's voltage meter in the next couple of days to test the 12v, and most likely replace it in the near future regardless of what I find (partially because I have no idea how old it is), but being a "hybrid noob", is there anything else I or a tech should look at or be aware of? Coming from a lifetime of gas engines, everything about this screams "dead 12v battery" to me, but the fact that it started after a few minutes of waiting for...something to charge the battery enough to start has me wondering if there is another problem somewhere else? :confused::confused: The fact it was cool this morning seems to debunk my "hot car" theory from the other day, so i'm a bit stumped!

    Thanks in advance for any help!

    Quick edit before anyone asks: first thing I did both times was check to see if I left any headlights or dome lights on overnight, and both times everything was switched off, so that isn't the issue here
     
    #1 brianhjazz, Jul 30, 2023
    Last edited: Jul 30, 2023
  2. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

    Joined:
    May 12, 2018
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    Location:
    Pennsylvania
    Vehicle:
    2018 Prius c
    Model:
    Two
    That behavior is a bit weird. Most modern cars draw a little bit of power even when "off." This is mostly so that one module can remain awake to listen for the radio signal to unlock the doors.

    When you open the drivers' door of a Prius c, there is an immediate spike in the 12v power load, because the brake booster starts charging its pressure accumulator.

    A low or dying automotive 12v battery (lead acid and subvariants) are known to recover somewhat after they've been drawn low and then allowed to rest without a load.

    None of this automatically adds up to the behavior you are describing, but potentially could allow for it.

    First, check the terminal connections on the battery to make sure they are tight. A loose or corroded terminal won't help, and in the case of a loose connection temperature can play a part.

    The 2012 model uses a battery with tapered terminals, wider at the bottom. To securely (re)connect one, you loosen the bolt on the clamp, set it over the battery terminal and then gently tap it downwards (the plastic handle of a screwdriver works great) until the taper is starting to force the clamp wider. Then tighten the clamp bolt.
     
    #2 Leadfoot J. McCoalroller, Jul 30, 2023
  3. brianhjazz

    brianhjazz New Member

    Joined:
    Jul 30, 2023
    2
    0
    0
    Location:
    MN, USA
    Vehicle:
    2012 Prius c
    Model:
    Two
    I wondered if maybe the connections were just loose as well. I will take a look at them later today and report back.

    Also, I do know that things like the car's computer (clock, etc) use battery power when the car is off, but are there any other components that could be drawing power that shouldn't be? I know anything is theoretically possible, just want to rule out any potential "common Prius issues" that I might not know about
     
    #3 brianhjazz, Jul 30, 2023
  4. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

    Joined:
    May 12, 2018
    7,427
    6,913
    1
    Location:
    Pennsylvania
    Vehicle:
    2018 Prius c
    Model:
    Two
    There are cases where the smart key system thinks it can detect your keyfob and wakes up another couple of modules in the car in anticipation of you getting in and driving away. If the keyfob is stored near enough to the car, this can repeat endlessly and drain more power.

    But if you have the Two model like mine, then you're using a regular metal twist key and the car doesn't even have those extra wireless modules.

    I've heard of CDs getting stuck in the stereo and the little ejector motor never ever gives up trying, which can flatten the battery given long enough. Ours was too new to have a CD player but your 2012 may have one.

    There have been complaints about the actual door lock actuators, the solenoids themselves that flip the mechanical locks. I have read that Toyota OEM replacements are expensive while no-name knockoffs are cheap and easy to find. Several people have reported that there doesn't seem to be a difference in quality between them. We haven't had that problem with our car yet, so I can't offer anything first-hand.

    That's all I can think of for "common" problems.

    The classic technique for chasing a phantom power drain is to hook up an ammeter to read the current load, then extract a fuse from the fuse box and look for a change in the load. If none is observed, that fuse goes back in and another comes out until you see the change. Then you go to the wiring diagram to figure out everything served by that fuse and check those individual components.
     
    #4 Leadfoot J. McCoalroller, Jul 30, 2023
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